It is known to process chirp RF signals by impressing them upon a laser beam using an acousto-optical modulator, and then optically processing the signal, utilizing adaptive techniques to operate at frequencies free of interference.
A system using adaptive spread spectrum technology is shown in U.S. Pat. No. 4,679,048, issued to R. W. Brandstetter et al on July 7, 1987 and assigned to a common assignee.
It is also known to optically process RF signals by optically notching unwanted spectral components from a laser beam modulated with the RF signal. Such a technique is illustrated in U.S. Pat. No. 4,522,466, issued to C. E. Lindig et al on June 11, 1985 and assigned to a common assignee.
The present invention features a novel optical processing of a broad band RF signal utilizing the optical notching technique depicted in U.S. Pat. No. 4,522,466.
The invention can also be used in any system where an RF chirp signal modulates a laser beam by means of an acousto-optic modulator. The present invention, therefore, could also be applied to the system shown in U.S. Pat. No. 4,679,048.
In this invention, the self-focusing optical properties of the general class of chirp signals emanating from an acousto-optic modulator as a modulated laser beam can be utilized to compress the chirp. The compressed chirp is then advantageously combined with a weighted local oscillator beam to accomplish the decoding process. The two beams, the chirp (signal) beam and the local oscillator beam with the weighting function impressed thereon, are combined and fed to a photomixer to obtain the decoded signal (compressed and weighted).
The weighting function can be utilized in the form of a programmable gradient density filter, which is modulated by a programmed noncoherent write-light source having a different wavelength than the laser beam.
In this fashion, nonlinear or discontinuous chirp signals can be decoded, thus providing a system which can adapt to changes in the chirp format in order to avoid interference.